Tool for electrolytic drilling of holes

ABSTRACT

A TOOL FOR USE IN THE ELECTROLYTIC DRILLING OF HOLES, COMPRISING A SELF-SUPPORTING TUBE OF ELECTRICALLY INSULATING MATERIAL HAVING AN OPERATIVE FREE OPEN END, AN ELECTRICALLY CONDUCTING WIRE WHICH EXTENDS ALONG THE BORE OF THE TUBE, WITH CLEARANCE BETWEEN THE WIRE AND THE BORE, TO PERMIT FLOW OF ELECTROLYTE, AND A SUPPORT WHICH FIXES THE WIRE AXIALLY RELATIVELY TO THE TUBE SO THAT A FREE END OF THE WIRE IS WITHIN THE TUBE AND SPACED AXIALLY FROM THE FREE END OF THE TUBE, AND WHICH PROVIDES FOR ELECTRICAL CONNECTION TO THE WIRE, AND FOR SUPPLY OF ELECTROLYTE TO THE CLEARANCE.

April 2, 1974 D. SAMSON 3,801,489

I TOOL FOR ELECTROLYTIC DRILLING OF HOLES Filed July 7 1971 4 Sheets-Sheet 1 April 2, 1974 D. SAMSON TOOLFOR ELECTROLYTIC DRILLING OF HOLES 4 Sheets-Sheet 2 Filed July 7 1971 April 2, 1974 D. SAMSON TOOL FOR ELECTROLYTIC DRILLING OF HOLES Filed July 7 1971 4 Sheets-Sheet 5 34 35 E /28 33 1 X RX &7

30 29 r y I April 2, 1974 D. SAMSON 3,801,489

TOOL FOR ELECTROLYTIC DRILLING OF HOLES Filed July 7, 1971 4 Sheets-$heet 4 US. Cl. 204-284 6 Claims ABSTRACT OF THE DISCLOSURE A tool for use in the electrolytic drilling of holes, comprising a self-supporting tube of electrically insulating material having an operative free open end, an electrically conducting wire which extends along the bore of the tube, with clearance between the wire and the bore, to permit flow of electrolyte, and a support which fixes the wire axially relatively to the tube so that a free end of the wire is within the tube and spaced axially from the free end of the tube, and which provides for electrical connection to the wire, and for supply of electrolyte to the clearance.

In the electrolytic drilling of holes, an elongated tubular tool is fed axially into a workpiece while at the same time a flow of electrolyte is maintained through the tool, and an electric current is maintained between the tip of the tool and the workpiece. The rate of feed is matched to the rate at which metal is removed from the work at the bottom of the hole by electrolytic action. The arrangement must be designed to avoid any short circuit from the tool to the work.

The most common form of tool is a tube of metal, coated externally with insulation.

However, we have found that it is difiicult or impossible to employ a metal tube coated with insulation, in situations where the diameter of the hole is to be of United States Patent the order of 0.020 inch (0.05 mm.) or less. It isnecessary for the external diameter of the metal tube to be less than the diameter of the hole by, an amount sufficient to accommodate an effective thickness of insulation, plus trolytic drilling of holes comprises a self-supporting tube is very expensive in some materials, and it is very ditlicult to draw so small a tube. Furthermore, such a small metal tube has very little strength.

According to this invention, a tool for use in the electrolytic driling of holes comprises a self-supporting tube of electrically insulating material having an operative free open end, an electrically conducting wire which extends along the bore of the tube, with clearance between the wire and the bore, to permit flow of electrolyte, and a support which fixes the wire axially relatively to the tube so that a free end of the wire is within the tube and spaced axially from the free end of the tube, and which provides for electrical connection to the wire, and for supply of electrolyte to the clearance.

The preferred insulating material is glass.

In carrying out the present invention, for given diameter of hole, the external diameter of the tube can be greater than the tube of an equivalent metal tool, because there is no layer of insulation.

We have found that glass can be made in suitable sizes, and sufiiciently straight, and it has mechanical properties which are superior to metal, in particular a higher yield point.

The wire, which serves as the electrode, does not require to contribute significantly 'to the mechanical strength of the tool.

3,801,489 Patented Apr. 2, 1974 Preferably the tube and the wire are of circular cross section and the internal diameter of the tube is about /3, and the diameter of the wire is about /s, of the external diameter of the tube.

The fact that the wire terminates upstream of the free end of the tube is essential, in order to obtain flow conditions such that metal is removed in a regular manner from the work, which is essential if the hole is to be reasonably straight and reasonably uniform in diameter.

The termination of the wire upstream of the free end of the tube ensures that the electrolyte can emerge from the free end of the tube as a coherent stable stream, despite the fact that the wire may be eccentric in the tube or may move laterally in the tube.

Preferably the distance axially between the free end of the wire and the free end of the tube is between 2 and 20 times the internal diameter of the tube.

The invention is of particular value in tools for making a plurality of holes.

One example of a multiple tool embodying the present invention, and of apparatus in which it is used, is shown in the accompanying drawings, in which:

FIG. 1 is an elevation of the apparatus as a whole.

FIG. 2 is a section on the line II-II in FIG. 1 drawn to an enlarged scale.

FIG. 3 is a section on the line IIIIII in FIG. 2.

FIG. 4 is a further enlarged detail of FIG. 2.

Referring to FIG. 1, a work table 10 is arranged to be raised or lowered relative to a tool 11 by means of a screw 12 through which the table is connected to a frame 13. The tool includes a shank 14 by which it is connected to the frame and through which an electrolyte supply pipe 15 is fed to the tool. A workpiece is indicated at 16, and the tool and the workpiece are connected electrically to the respective poles of current supply 17 as shown.

Details of the tool are shown in FIGS. 2 and 3. The tool comprises a plurality of electrodes having the form of wires 20 situated in tubes 21. The wires 20 terminate upstream of the ends of the tubes 21 by a distance 22 (FIG. 4).

A plurality of parallel wires and tubes are provided for the purpose of drilling a plurality of parallel holes. p The tool comprises a manifold 23 supporting the tubes 21 and through which these tubes are supplied with the electrolyte from the tube 15. The tubes 21 are located in a slot 24 (FIG. 4) in a nose 25 of the manifold and are secured in position by means of a resin.

The manifold, which is made of a metal which is resistant to corrosion in the electrolyte, e.g. 18% chromium stainless steel, comprises a base 26 to which the nose 25 is secured, and a housing 27 removably secured to the base. The base has secured thereto a clamp 28 comprising a pair of jaws 29, 30 for gripping the Wires 20. The jaw 30 is pivoted to the jaw 29 on an axis such that the clamping force is normal to the length of the wires, and a screw 31, located in a pivotal stirrup 32, is provided for locking and releasing the clamp. The jaw 29 is supported for sliding motion in the direction of the length of the wires by a guide 33. A screw 34, nut 35, and spring 36 are arranged so that rotation of the nut in one direction raises the clamp 28 against the spring 36. In this way the longitudinal position of the wires 20 in the tubes 21 can be adjusted during the initial assem bly of the wires and before the housing 27 is attached to the base 26.

In operation, the tool and workpiece are mounted in the correct machining relationship on the frame 13 and table 10 respectively, and the table is raised by means of the screw 12 to perform the feed motion between the tool and workpiece in a manner well understood per se.

As an example, for the purpose of drilling holes of 0.010 inch (0.25 mm.) diameter, the diameter ofthe electrode is 0.0015 inch (0.04 mm.), and the internal and external diameters of the tubes are 0.005 and 0.0075 inch (0.13 and 0.19 mm.) respectively. The feed rate for such holes would be 0.050 inch (1.30 mm.) per min. The unsupported length of the tubes is 0.4 inch mm.) so that the maximum depth of hole which can be drilled is of the order 0.35 inch (9 mm.), to avoid secondary machining problems which can arise if a conducting path is established between the nose 25 and the workpiece. The voltage applied between the wires and the workpiece is about 100 volts, and the electrolyte is supplied to the manifold at room temperature at about 100 lbs/sq. in (700K N/m.

A number of practical problems arise in the performance of the invention due to the extremely small diameters and the close pitching of the holes to be drilled.

The greater the axial distance 22 between the free end of each wire 20 and the free end of the respective tube 21, the more uniform the rate of machining and the greater the reliability and consistency of the process. If the distance 22 is too small, any slight movement of the wire transversely or longitudinally in the tube will have undesirably large effects on the stability and control of the process. For example, a small change in the concentricity ofthe wire position in the tube, due to flutter induced by the flow of electrolyte, has a much more pronounced effect on the uniformity of the electrolyte flow at the mouth of the tubes that when the distance 22 is greater. Also any variation in the length of the wire due to, say, straightening from a somewhat bent state, would produce an undesirably large percentage variation in the gap between electrode and workpiece. We find that the distance 22 must not be smaller than about 2 times the internal diameter of the tube.

If the distance 22 is too great, then an undesirably large voltage is required for machining, with accompanying complications arising from safety requirements. Thus the distance 22 should not be larger than is required for process stability, and a dimension of about to times the internal diameter of the tube has been found to be the maximum necessary for practical purposes.

It will be clear that the distance 22 is independent of the length of the tubes 21, and no change in voltage, for example, is necessary if the length of the tubes and wires is changed to suit different depths of drilling.

The provision of the distance 22 makes it possible for electrolyte flow to establish a coherent stream at the electrolyte pressure used. In this way there is overcome the disturbing effect on uniformity of flow which would be produced by the wire not being perfectly concentric with the tube or by any vibration of the wire in operation. Nevertheless it is desirable for the wires to have as much stiffness as possible to diminish flutter in the tube and to diminish the possibility of bending these fine wires when they are being mounted in the tool. To this end the wires are preferably made of platinum containing 10% rhodium, although they could be made from any noble metal, provided they can be made sufficiently stiff to be threaded up the tubes, and as a final operation before assembling the wires in the tool they are rolled between glass plates for the purpose of straightening and work hardening them.

In the example described above, to arrange the wire in the tool, the clamp is opened and the wires are individually fed into the tubes 21 from the free ends thereof, and when all the wires have been so fed the clamp is tightened, gripping the wires between an anvil 40 on the clamp body and the jaw 29 of the clamp. In an alternative method, the wires may be spot welded to the anvil, and the clamp jaw 29 can be eliminated.

The wires are then cut substantially flush with the ends of the tube and are thereafter withdrawn into the tubes by 4. shifting the clamp by means of the nut 35, so as to vide the same distance 22 for all the wires.

The distance 22 can also be produced by electrolytic erosion. At first the wires are passed into the tubes and are held by the clamp Welding as described above. Short lengths of wire are allowed to project outside the ends of the tubes and are trimmed substantially flush with these ends. Thereafter a flow of a suitable electrolyte is intro duced down the tubes and an electric circuit is established between the wires and a common electrode (not shown) arranged below the tool, so that the wires are elecrolytically eroded, i.e. the current flow is in the opposite sense to that used when drilling the workpiece. For a platinumrhodium wire a suitable electrolyte would be 20% potassium cyanide. The common electrode cn be a carbon block placed on the work table. The table is adjusted so that the tips of the tubes are spaced a short distance from the surface of the carbon block. The main advantage of this process is that different wires 21 are eroded to different distances 22 due to the effect of differences in the internal diameters of the tubes, and this in turn produces uniformity in the rate of machining of the holes to be drilled and in the diameters of these holes.

The upper limit to the diameter of the wire is determined by the need for an adequate clearance between the wire and the tube for flow of electrolyte. The lower limit to the diameter of the wire is determined by the need to avoid melting of the wire by the electric current. Moreover the thinner the wire, the more difficult it is to manipulate into position.

The invention overcomes the difi'iculty of making drillalthough not limited to this range, up to a length to diameters of the order mentioned.

The invention thus provides a stable process for drilling holes of the order of 0.004 to 0.020 inch (0.10 to 0.50 mm.) diameter, using tools having an external diameter in the range 0.0035 to 0.017 inch (0.09 to 0.43 mm.), although not limited to this range, up to a length to diameter ratio of the order of 100, with a substantially constant voltage of the order of volts.

In addition the diameter of the holes drilled can be varied using the same drill by varying the feed rate and voltage.

The invention is also applicable to drilling of shaped holes by having tubes of shaped cross-section.

Instead of glass one can use aluminum oxide (synthetic sapphire) which, unlike glass, is ont attacked by fluorides in the electrolyte, or silicon oxide (quartz) which, like the aluminium oxide, is stronger than glass.

The pitch of the tubes (i.e. the distance between their axes) has a lower limit determined by the surface tension of the electrolyte and the length/diameter ratio of the tubes. If the pitch is too small, the tubes tend to deflect from their parallel relationship and to adhere together. A pitch of 0.040 inch (1 mm.) has'been found to be the minimum that can be maintained without this adhesion, when using 20% nitric acid as the electrolyte and using the dimensions of tubes as described in the example above. But if a surface tension reducing agent is added to the electrolyte, the pitch can be reduced. A suitable agent is dodecylbenzinesulphonic acid.

Provision may be made to play a jet of air on to the tubes to prevent the surface tension effects mentioned above during drilling, and to prevent a fountain of electrolyte from the holes from establishing a conducting path between the nose and the workpiece. A further means of preventing this latter effect would be to make the nose from an insulating material or to coat it with such-a material.

I claim: i

1. A tool for use in the electrolytic drilling of holes, comprising a self-supporting tube portion of electrically insulating material having an operative free open end and defining a bore of uniform cross section throughout its length and communicating directly with surrounding space proat said free open end, an electrically conducting wire which extends along said bore, with clearance between the wire and the bore, to permit flow of electrolyte, and a support which fixes the wire axially relatively to the tube portion so that a free end of the wire is within the bore and spaced axially from the free open end by a distance between two times and 20 times the internal diameter of the tube, and which provides for electrical connection to the wire, and for supply of electrolyte to enter the clearance and emerge to surrounding space at said free tube open end.

2. A tool according to claim 1, in which the tube portion is of circular cross section, with an external diameter in the range 0.0035 to 0.017 inch.

3. A tool according of claim 1, in which the tube portion and the wire are each of circular cross section and the internal diameter of the tube is about and the diameter of the wire is about /5, of the external diameter of the tube.

4. A tool according to claim 1, in which the tube is of glass.

5. A tool according to claim 1, in which the wire is of platinum containing rhodium.

6. A multiple tool for use in the simultaneous electrical drilling of a plurality of holes, comprising a plurality of self-supporting tubes of electrically insulating material, each having an operative free open end and defining a bore of uniform cross section throughout its length and communicating directly with surrounding space at said free open end, a support which fixes the tubes with their bores parallel to each other, and a plurality of electrically conducting wires each of which extends along the bore of a respective one of the tubes, with clearance between the wire and the bore, to permit flow of electrolyte, the support including a manifold to which the tubes are attached, a clamp by which the wires are gripped, and means for adjusting the clamp relatively to the manifold to shift the wires along the bores of the tubes, enabling fixing of each wire axially relatively to the respective tube so that a free end of the wire is within the tube and spaced axially from the free open end of the tube, by a distance between two and 20 times the internal diameter of the tube, and the support also providing for electrical connection to the wires, and for supply of electrolyte to enter the clearances and emerge to surrounding space at said free open end.

References Cited UNITED STATES PATENTS 3,409,534 11/1968 Andrews et al. 204-143 M 3,384,567 5/ 1968 Andrews et al 204 ECM 3,467,592 9/ 1969 -Eisberg et a1 204 ECM 3,403,085 9/1968 Berger et al 204- ECM 3,352,770 11/ 1967 Crawford et al 204 ECM FOREIGN PATENTS 979,968 1/1965 Great Britain 204 ECM FREDERICK C. EDMUNDSON, Primary Examiner US. Cl. X.R. 

